Recently, sport-utility vehicles and minivans have become increasingly popular among automobile consumers. Such vehicles include large cargo areas that provide increased hauling capability relative to conventional passenger vehicles. To maximize the accessibility to these cargo areas, many vehicles are equipped with a liftgate or cargo door located at the rear of the vehicle. Typically, these liftgates are pivotally attached by hinges to the top of the cargo opening. The cargo opening is defined by the roof and rear pillars of the vehicle. To gain access to the cargo area, these liftgates commonly pivot upwardly and outwardly from the cargo opening.
In conventional liftgates, pneumatic "actuators" or "cylinders" containing compressed gas are provided on each side of the liftgate. Each pneumatic actuator is attached at one end to the liftgate and at another end to the corresponding pillar of the vehicle. When the liftgate is closed and latched, the pneumatic actuators are contracted and the gas within the actuators is compressed. When the liftgate is unlatched, the stored energy provided by the compressed gas in the pneumatic actuators forces the liftgate to open partially, thereby releasing the liftgate from the lock. The liftgate must then be manually lifted while the pneumatic actuators continue to exert an outward force on the liftgate that assists the manual opening of the liftgate. Eventually, the liftgate is manually lifted to a position where the geometric relationship of the pneumatic actuators relative to the liftgate is such that the moment arms of the pneumatic actuators are sufficiently large to enable the actuators to take over lifting the liftgate and retain the liftgate in a fully opening position. More particularly, the pivoting dynamics of the liftgate are a function of:
1) the force exerted by the pneumatic actuators (F.sub.a); PA1 2) the distance between the vector force of the pneumatic actuators and the liftgate pivot axis (d); PA1 3) the weight of the liftgate (F.sub.g); and PA1 4) the distance of the liftgate center of gravity and the liftgate pivot axis (D). The product of F.sub.a and d equals the moment force of the actuators. The product of F.sub.g and D equals the moment force of gravity acting on the liftgate. It should be appreciated that the force due to gravity and the force exerted by the actuators vary relative to the position of the liftgate.
It should be appreciated to one skilled in the art that the above conventional design includes an "overcenter condition." This condition provides a "center" position in which the moment exerted by the actuators is equal to the moment exerted by the weight of the liftgate. At a position above the "center" position, the upward moment forces the liftgate open. At a position below the "center" position, the downward moment overcomes the upward moment, thereby allowing the liftgate to generally close. It should be appreciated that due to the weight of conventional liftgates, opening the liftgate prior to the "center" position may be awkward and difficult. Similarly, due to the force exerted by the actuators, closing the liftgate prior to the "center" position may further be awkward and difficult.
Attempts have been made to provide hydraulic and/or cable driven systems to automatically open or close the liftgate of a vehicle. However, various disadvantages are associated with these types of systems. For example, but not limited to, the hydraulic type system requires expensive and cumbersome hydraulic pumps and actuators. Furthermore, these pumps and actuators are typically difficult to install due to their size and complexity and are generally located at a distance away from the liftgate mechanism. Likewise, cable type systems may be unreliable due to the exposure of the cable and pulleys to environmental contamination and the like.
Accordingly, there exists a need in the relevant art to provide a powered liftgate assembly that is capable of remotely opening and closing the liftgate of a vehicle. Furthermore, there exists a need in the relevant art to provide a powered liftgate assembly that is capable of being manually overridden by an operator. Moreover, there exists a need in the relevant art to provide a powered liftgate assembly that overcomes the disadvantages of the prior art.